Device for determination and/or monitoring of the volumetric and/or mass flow of a medium and having coupling element including two element portions

ABSTRACT

A clamp-on ultrasonic flow measuring device for determining volume and/or mass flow rate of a medium in a containment. The clamp-on ultrasonic measuring device is of low temperature sensitivity. To this end, the coupling element, through which the ultrasonic measuring signals are coupled into, and/or out of, the containment, has at least two element portions, which are embodied and/or arranged in such a manner that the predetermined in-coupling angle into the containment and/or the predetermined out-coupling angle out of the containment are/is approximately constant over an extended temperature range.

FIELD OF THE INVENTION

The invention relates to an apparatus for determining and/or monitoringthe volume and/or mass flow rate of a medium in a containment,especially in a pipe. The apparatus includes: at least one ultrasonictransducer, which emits and/or receives ultrasonic measuring signals;associated with the ultrasonic transducer, a coupling element, via whichthe ultrasonic measuring signals are coupled into, and out of, thecontainment at a predetermined in-coupling/out-coupling angle; and acontrol/evaluation unit, which, on the basis of the measuring signals,or on the basis of measurement data derived from the measuring signals,determines the volume and/or mass flow rate of the medium flowing in thecontainment.

BACKGROUND OF THE INVENTION

Ultrasonic flow measuring devices are applied often in process andautomation technology. They make possible contactless determination ofthe volume and/or mass flow rate of a medium in a pipeline.

Known ultrasonic flow measuring devices work either by the Dopplerprinciple or the travel-time-difference principle. In the case of thetravel-time-difference principle, the different travel times of theultrasonic measuring signals in the direction of flow, and counter tothe direction of flow, of the medium are exploited. To this end, theultrasonic measuring signals are alternatingly issued, respectivelyreceived, in the direction of flow, and counter to the direction offlow, of the medium. On the basis of the travel-time-difference of theultrasonic measuring signals, the flow velocity can be determined, and,with that and known diameter of the pipe, the volume flow rate of themedium, or, with known density, the mass flow rate of the medium.

In the case of the Doppler principle, ultrasonic measuring signals ofpredetermined frequency are coupled into the flowing medium. Theultrasonic measuring signals reflected in the medium are evaluated. Onthe basis of a frequency shift occurring between the ultrasonicmeasuring signal which was coupled into the medium and the reflectedultrasonic measuring signal, likewise the flow velocity of the medium,or the volume and/or mass flow rate, can be determined.

The use of flow measuring devices working according to the Dopplerprinciple is only possible, when present in the medium are air bubblesor impurities, on which the ultrasonic measuring signals are reflected.Thus, use of ultrasonic flow measuring devices using the Dopplerprinciple is rather limited, compared to ultrasonic flow measuringdevices using the travel-time-difference principle.

With respect to types of measuring devices, a distinction is drawnbetween ultrasonic flow measuring pickups, which are inserted into thepipeline, and clamp-on flow measuring devices, where the ultrasonictransducers are pressed onto the pipeline externally by means of a clampconnection. Clamp-on flow measuring devices are described, for example,in EP 0 686 255 B1, U.S. Pat. No. 4,484,478 or U.S. Pat. No. 4,598,593.

In the case of the two types of ultrasonic flow measuring devices, theultrasonic measuring signals are radiated at a predetermined angle into,and/or received from, the pipeline, or measuring tube, as the case maybe, containing the flowing medium. In order to achieve an optimumimpedance matching, the ultrasonic measuring signals are coupled into,or out of, the pipeline via a lead-in member, or a coupling wedge, asthe case may be. Principal component of an ultrasonic transducer is atleast one piezoelectric element, which produces and/or receives theultrasonic measuring signals.

The ultrasonic measuring signals produced in a piezoelectric element areled via the coupling wedge, or lead-in member, as the case may be, and,in the case of a clamp-on flow measuring device, through the pipe wall,into the liquid medium. Since the velocities of sound in a liquid and inplastic differ from one another, the ultrasonic waves are refracted atthe transition from one medium into the other. The angle of refractionat the transition from one medium into another medium is dependent onthe ratio of the velocities of sound c_(m), c_(n) in the two media n, m.

Mathematically, Snell's law can preferably be expressed according to thefollowing formula:c _(n)/sin α_(n) =c _(m)/sin α_(m)=const.  (1)where:c_(n) is the velocity of sound e.g. in the coupling wedge made, forexample, of plastic;c_(m) is the velocity of sound e.g. in the medium, which is, forexample, water;α_(n) is the angle between the sound path and the normal to the boundingsurface of the coupling wedge at the point where the ultrasonicmeasuring signal passes through the bounding surface; andα_(m) is the angle between the sound path and the normal to the boundingsurface of the medium at the point where the ultrasonic measuring signalpasses through the bounding surface.

With coupling wedges, or lead-in members, of plastic, among otherthings, a good impedance matching can be achieved; however, the velocityof sound in plastic has a relatively strong temperature dependence.Typically, the velocity of sound in plastic changes from about 2500 m/sat 25° C. to about 2200 m/s at 130° C. In addition to the change oftravel time of ultrasonic measuring signals in the plastic of thecoupling wedge brought about by temperature, the direction ofpropagation of the ultrasonic measuring signals in the flowing mediumalso changes. Both changes, in the case of an ultrasonic flow measuringdevice operating according to the travel time difference method,naturally act unfavorably on the accuracy of measurement. Added to thisis the fact that the propagation velocity exhibits, in certain media,likewise a strong temperature dependence.

For coping with the temperature dependence of the coupling wedges, it isknown from WO 02/39069 A2 to construct the coupling element out of aplurality of segments in the form of circular arcs. Preferably, thesegments are made of metal. The individual segments are arrangedseparated from one another and they extend from a contact plane, whichfaces the piezoelectric element, out to a base plate, which is connectedwith the pipe wall. The length of the individual segments is, in suchcase, so measured, that the ultrasonic measuring signals are radiatedand received at a predetermined angle at the base plate. This embodimentis, however, relatively complex.

SUMMARY OF THE INVENTION

An object of the invention is to provide a clamp-on ultrasonic measuringdevice, whose measuring accuracy is relatively insensitive totemperature changes of the medium and/or of the environment.

The object is solved by providing the coupling element with at least twoelement portions, which are embodied and/or arranged in such a mannerthat the predetermined in-coupling angle into the containment, or thepredetermined out-coupling angle out of the containment, isapproximately independent of the temperature of the coupling wedge overan extended range of temperature. “Extended range of temperature” meanshere at least the temperature range of about 0° C. to 130° C.

An advantageous further development of the apparatus of the inventionconcerns the case where the velocity of sound in the medium beingmeasured itself exhibits a relatively strong temperature dependence andwhere the temperature of the medium changes in step with the temperatureof the coupling wedge. In this case, in agreement with Snell's law, thein-coupling, or out-coupling, angle of the ultrasonic measuring signalsis determined also by the temperature dependence of the velocity ofsound in the medium. In order to keep the in-coupling angle into themedium, or the out-coupling angle out of the medium, in accordance withthe invention, essentially constant over an extended range oftemperature, the materials and the dimensions of the at least twoelement portions of the coupling element are so selected that, ineffect, no change of the incidence/reflection angle occurs, which wouldnegatively influence the measurement result within desired tolerancelimits. In the case of this solution, one is dealing with amedium-specific solution.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in greater detail on the basis ofthe drawings, the figures of which show as follows:

FIG. 1 a schematic drawing of a clamp-on ultrasonic flow measuringdevice in two-traverse mode;

FIG. 2 a longitudinal section through a form of embodiment of theultrasonic transducer of the invention; and

FIG. 3 a graphic presentation of the in-coupling/out-coupling angle as afunction of temperature, with and without compensation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred embodiment of the apparatus of the invention, thecoupling element comprises at least two coupling wedges, which aresuccessively traversed by the ultrasonic measuring signals. Preferably,the coupling wedges are made of plastics with different soundvelocities.

The element portions, or the coupling wedges, as the case may be,comprise, in an advantageous embodiment of the apparatus of theinvention, different materials, with the materials being selected suchthat temperature-related changes of sound velocity, or the index ofrefraction, of a first element portion, or a first coupling element, asthe case may be, are at least approximately compensated bytemperature-related changes of sound velocity, or index of refraction,of at least a second element portion, or a second coupling wedge, as thecase may be. Preferably, the compensation occurs over as great atemperature range as possible.

In an alternative embodiment of the apparatus of the invention, aplurality of element portions, or a plurality of mutually connectedcoupling wedges, as the case may be, of different materials areprovided, with the materials being so selected that temperature-relatedchanges of sound velocity, or index of refraction, of the medium andtemperature-related changes of the sound velocities, or indices ofrefraction, in the at least two element portions, or coupling wedges,essentially mutually compensate one another.

With this embodiment, the influence of temperature fluctuations of themedium on the in-coupling, or out-coupling, angle can be directlyeliminated, or the effects can be kept so small, that the measurementaccuracy is only insignificantly degraded.

In an advantageous embodiment of the apparatus of the invention, it isprovided that the path lengths, which the ultrasonic measuring signalstrace in the element portions of the coupling wedges, or the lead-inmembers, are so selected, that the sum of the corresponding traveltimes, which the ultrasonic measuring signals require for traversing theelement portions, is at least approximately constant over apredetermined temperature range. This is achieved preferably by theappropriately selected dimensioning of the element portions. Thisembodiment assures that, almost independently of temperature changes,always the maximum signal amplitude of an ultrasonic measuring signal isreceived from each ultrasonic transducer. More or less complexreadjustments of the ultrasonic transducers on the pipe due totemperature changes in the sensors are, consequently, not required.

FIG. 1 is a schematic presentation of a clamp-on flow measuring device 1in two-traverse mode 10. The flow measuring device 1 determines volumeflow rate and/or mass flow rate of the medium 2 in the pipe 7 using theknown travel-time-difference method.

Essential components of the clamp-on ultrasonic flow measuring device 1are the two ultrasonic transducers 3, 4 and the control/evaluation unit9. The two ultrasonic transducers 3, 4 are attached to the pipe 7 bymeans of a securement apparatus not separately shown in the figure.Appropriate securement apparatuses are sufficiently known in the stateof the art and are also available from the assignee. Medium 2 flowsthrough pipe 7 of predetermined inner diameter di in the streamdirection S.

An ultrasonic transducer 3; 4 includes, as essential components: Atleast one piezoelectric element 5; 6, which produces and/or receives theultrasonic measuring signals; and a coupling wedge, or lead-in element,11; 12. The ultrasonic measuring signals are coupled via the couplingwedges 11, 12 into, and out of, the pipe 7 containing the flowing medium2. The coupling wedges 11; 12 determine the directions of the ultrasonicmeasuring signals into and out of the pipe and medium; additionally,they serve for optimizing the impedance matching of the ultrasonicmeasuring signals at the transition into and out of the pipe 7.

The two ultrasonic transducers 3, 4 are positioned at a separation Lfrom one another, with the separation L being selected such that an ashigh as possible energy fraction of the ultrasonic measuring signalssent from an ultrasonic transducer 3; 4 is received in the respectiveother ultrasonic transducer 4; 3. The optimum positioning depends on anumber of different system and/or process variables. These system andprocess variables include, for example, the inner diameter di of thepipe 7, the thickness of the pipe wall 8, the velocity of sound c₃ inthe material of which the pipe is made, or the velocity of sound C₄ inthe medium 2. Additionally to be considered is that the velocities ofsound in the different materials, such as the materials of the couplingwedge, pipe wall and medium, exhibit various degrees of temperaturedependence.

In the illustrated case, the separation L of the two ultrasonictransducer 3, 4 is so selected that the ultrasonic measuring signals,which, according to the travel-time-difference method, are alternatelyemitted and received from and by the two ultrasonic transducers 3, 4,propagate via the sound path SP in the pipe 7 containing the flowingmedium 10. Sound path SP exhibits two traverses, thus two crossings ofthe pipe 7. The traverses can be diametral or chordal.

FIG. 2 shows, in section, a form of embodiment of the ultrasonictransducer 3; 4. The coupling element 11; 12, according to theinvention, is composed of at least two element portions 13, 14, whichare successively traversed by the ultrasonic measuring signals emittedfrom, or received by, the piezoelectric element 5; 6.

Consider first the case in which the electronic measuring signals arecoupled into, or out of, pipe 7 on the basis of element portion 13alone. The in-coupling, out-coupling angle is essentially determined bythe geometry of the element portion 13, i.e. the geometry of elementportion 13 is so chosen, that as much energy as possible passes throughthe boundary surface between the element portion 13 and the pipecontaining the flowing medium 2. The in-coupling and out-coupling of ahigh energy fraction of the ultrasonic measuring signal is of decisiveimportance for a good measurement accuracy. In order to achieve thereliable measurement results over any long period of time, it is,moreover, important that a determined, optimizedin-coupling/out-coupling angle also subsequently remains constant. Asindicated every deviation from the predetermined value leads to adegrading of the measurement accuracy. The permanent keeping of theincidence/reflection angle of the measuring signal is especiallyproblematic, because velocities of sound in the different materialschange in varying degrees as a function of temperature.

This is where the solution of the invention comes in: By adding a secondelement portion 14, whose sound velocity differs from the sound velocityof the first element portion 13, it becomes possible to compensate, atleast approximately, and, in the ideal case, completely, the temperaturedependence of the coupling element 11; 12, or lead-in member. Of course,the coupling element 11; 12 of the invention can also be constructed ofmore than two element portions 13, 14. These can be embodied such thatthe temperature-dependent angle of refraction of an individual elementportion 13; 14 is opposed to the sum of the temperature-dependent anglesof refraction of all remaining element portions of the coupling element11; 12.

In principle, equally significant is the case that, in addition to thetemperature dependence of the sound velocities of the coupling elements11, 12, also the velocity of sound in the medium 2 has a strongtemperature dependence. By way of example, let water be here the medium2. In such an application, the coupling element 11; 12, composed of atleast two element portions 13, 14, is so embodied that it compensates,at least approximately, the influence of temperature changes of thewater on the in-coupling, or out-coupling, angle of the ultrasonicmeasuring signals over an appropriately large temperature range.

In order to determine the suitable angles in the element portions 13, 14of the coupling element 11; 12, the sound path SP can be calculated fora temperature range or for individual reference temperatures (in theillustrated case: 25° C.) and the incidence/reflection angle in themedium to be measured, e.g. in water, kept at as constant a value aspossible. Additionally, also the entrance position into the medium 2 tobe measured and the exit position from the medium to be measured aredependent on the incidence/reflection angles in the element portions 13,14. In order to keep the temperature influence in the ultrasonictransducers as small as possible, the element portions 13, 14 aredimensioned such that the sum of the travel times of the ultrasonicmeasuring signals through the element portions 13, 14 of the ultrasonictransducers 3, 4 is constant over a wide temperature range.

Mathematically, the dependence of the velocity of sound c in a medium non temperature can be expressed to a first approximation according tothe following equation (2):c _(T,n) =c _(25° C.) +Δc·T  (2)

Reference value for the temperature change of the sound velocity isusually the velocity of sound in the medium n at 25° C. Δc in theformula represents the change of the sound velocity c as a function oftemperature T.

By successive application of Snell's law, the in-coupling/out-couplingangle ζ in the medium (n=4) flowing in the pipe can be calculated bymeans of the following formula:

$\begin{matrix}{\zeta_{T,4} = {a\;{\sin\left( {\frac{c_{T,4}}{c_{T,2}} \cdot {\sin\left( {\delta_{3} - {a\;{\sin\left( {{\frac{c_{T,2}}{c_{T,1}} \cdot \sin}\;\delta_{2}} \right)}}} \right)}} \right)}}} & (3)\end{matrix}$where:

T is temperature;

c(T,n) is the velocity of sound in the different materials, with theindices n=1 . . . 4 representing

1 the compensating wedge, i.e. the second element portion 14;

2 the coupling wedge, i.e. the first element portion 13;

3 the pipe wall 8;

4 the medium 2 flowing in the pipe 7;

δ₂ is the angle of the compensating wedge 14; and

δ₃ is the angle of the coupling wedge.

If the temperature of the medium is constant or if the change of thevelocity of sound in the medium can be neglected over the temperaturerange, then the following formula holds:

$\begin{matrix}{\frac{\sin\left( {\delta_{3} - {a\;{\sin\left( {{\frac{c_{T,2}}{c_{T,1}} \cdot \sin}\;\delta_{2}} \right)}}} \right)}{c_{T,2}} = {{const}.(T)}} & (4)\end{matrix}$where:T is temperature;c(T,1) is the velocity of sound in the compensation wedge, i.e. in thesecond element portion 14;c(T,2) is the velocity of sound in the coupling wedge, i.e. in the firstelement portion 13;δ₂ is the angle of the compensation wedge; andδ₃ is the angle of the coupling wedge.

FIG. 3 shows graphically how, by means of the solution of the invention,the influence of temperature on the in-coupling/out-coupling angle ζ inand out of the medium is approximately compensated. In particular, thecontinuous line represents the temperature dependence of thein-coupling/out-coupling angle ζ of the ultrasonic measuring signal intoand out of the medium 2 with compensation; the dashed line shows thecorresponding temperature dependence of the incidence/reflection angle θwithout the compensation of the invention. Δθ(T,3) represents thecorresponding angle change, which occurs in the case of a couplingelement 13 without additional compensation wedge 14. Δζ(T,4) is thedeviation versus temperature of ζ(T,4) relative to an angle of incidenceat 25° C. The measured medium 2 is water in the illustrated case. Thefirst element portion 13 is a plastic with a sound velocity c(25° C.,1)of 2668 m/s and Δc₁=−4.5 m/s/K. In the case of the second elementportion 14, such is a plastic with a sound velocity c(25° C.,2) of 2451m/s and Δc₂=−0.73 m/s/K. The curves show that, in the temperature rangefrom 0° C. to 100° C., by adding the second element portion 14 accordingto the invention (the compensation wedge), the temperature dependence ofthe angle ζ of incidence/reflection into/out-of the medium 2 isapproximately compensated. To an approximation, the angle ζ ofincidence/reflection in the medium 2 is constant over the entiretemperature range in which the ultrasonic flow measuring device 1 is, orcan be, used.

1. A clamp-on flowmeter for determining and/or monitoring the volumeand/or mass flow rate of a medium in a pipe, comprising: two ultrasonictransducers, which emit receive ultrasonic measuring signals; couplingelements associated with each of said ultrasonic transducers, via whichthe ultrasonic measuring signals are coupled into, and out of, the pipeat a predetermined in-coupling and out-coupling angle; and acontrol/evaluation unit, which determines the volume and/or mass flowrate of the medium flowing in the pipe on the basis of the travel-timedifference principle; wherein: said coupling element includes at leasttwo element portions, which are embodied and/or arranged in such amanner that the influence of temperature changes on a predeterminedin-coupling angle (ζ) into the containment and/or on a predeterminedout-coupling angle (ζ) out of the containment is approximatelycompensated in a predetermined, or extended, temperature range, and saidat least two element portions comprise coupling wedges or lead-inmembers, the dimensions of said at least two element portions, or thepath lengths, which the ultrasonic measuring signals travel in saidcoupling wedges or said lead-in members, are so selected that the sum ofthe corresponding travel times, which the ultrasonic measuring signalsrequire for passing through said coupling wedges or said lead-inmembers, is at least approximately constant over a predeterminedtemperature range.
 2. The apparatus as claimed in claim 1, wherein: thein-coupling/out-coupling angle (ζ) of the ultrasonic measuring signalsis determined also by the temperature dependence of the medium, said atleast two element portions of said coupling element are embodied and/orarranged in such a manner that the in-coupling/out-coupling angle (ζ),respectively, into the medium or out of the medium, is approximatelyconstant over an extended temperature range.
 3. The apparatus as claimedin claim 1, wherein: said at least two element portions are couplingwedges, which are passed through successively by the ultrasonicmeasuring signals.
 4. The apparatus as claimed in claim 1, wherein: saidat least two element portions, are made of different materials, whereinthe materials are so selected that changes of the velocity of sound (c2)in, or the index of refraction of, the material of a first elementportion, or of a first coupling wedge, caused by temperature changes areapproximately compensated for by changes of the velocity of sound (c1)in, or the index of refraction of, at least a second element portion, orof a second coupling wedge, caused by temperature changes.
 5. Theapparatus as claimed in claim 1, wherein: said at least two elementportions are made of plastic.
 6. The apparatus as claimed in claim 1,wherein: a plurality of element portions, of different materials areprovided; and the materials are so selected that changes of the velocityof sound in, or the index of refraction of, the medium caused bytemperatures changes, and changes of the velocities of sound in, or theindices of refraction of, said plurality of element portions, caused bytemperature changes, are approximately mutually compensated.